Organic Light Emitting Diode (OLED) devices include a thin film of electroluminescent organic material sandwiched between a cathode and an anode, with one or both of these electrodes being a transparent conductor. When a voltage is applied across the device, electrons and holes are injected from their respective electrodes and recombine in the electroluminescent organic material through the intermediate formation of emissive excitons.
In OLED devices, over 70% of the generated light is typically lost due to processes within the device structure. The trapping of light at the interfaces between the higher index organic and Indium Tin Oxide (ITO) layers and the lower index substrate layers is the major cause of this poor extraction efficiency. Only a relatively small amount of the emitted light emerges through the transparent electrode as “useful” light. The majority of the light undergoes internal reflections, which result in its being emitted from the edge of the device or trapped within the device and eventually being lost to absorption within the device after making repeated passes.
Light extraction films use internal nanostructures to avoid waveguiding losses within the device. While providing strong light extraction, internal nanostructures, comprising regular features such as photonic crystals or linear gratings, tend to produce diffraction-related variations in the angular and spectral distributions of the emitted light, which may not be desirable in final applications. Thus, a need exists for a light extraction film that simultaneously enhances the efficiency of light extraction via the nanostructures, while also reducing undesirable luminance and color angular non-uniformity in the light output.